Morphology & biology
The characteristic feature of the mackerel is its blue-green back with more or less oblique and parallel dark lines, while the flanks of the belly are silvery white. The zigzag pattern on their backs consists of 23 to 33 stripes. Before the strongly forked tail fluke, the mackerel has five small fin-approaches on both sides, that are called feather-leaflets or also “raftsels”.
The average size of the adult mackerel is 35 cm, but lengths up to 60 cm are possible. The average weight is 0.5 kg, but large mackerels can weigh up to 3 kg.
A biological characteristic of the mackerel is that they lack the swim bladder. This makes them very mobile and helps them to avoid predators such as the spiny dogfish, porbeagle sharks and tuna, dive to greater depths and quickly return to the surface. On the other hand, the mackerel has to stay in motion to avoid sinking.
The breeding season of the mackerel depends on the temperature of the water. At the end of the year, the sexually mature mackerel migrate to the spawning grounds. In March-April the mackerel spawn in the south of Ireland and in May-June in the west of Ireland and northwest of Scotland. The spawning takes place at a temperature of 12-13 ° C and a depth of 80 to 120 meters. The female lays 350 to 450,000 eggs in water, which are immediately distributed by the current in the sea. After five days the larvae hatch and migrate to the protected coastal areas to grow up there. After one year, the mackerel is about 25 cm tall and weighs about 100 g. At the age of approx. three years and a body height of 30 cm the mackerel become sexually mature. They can become up to 20 years old.
During the winter, the mackerels rest and take up no food. From spring onwards they feed on plankton. They have developed an impressive technique: In densely crowded swarms, the mackerels swim with wide open mouths through the sea. In this mouth to mouth formation they swim together and make it impossible for the plankton to escape. If a small oar-foot-cancer tries to avoid the mouth of a mackerel, it lands for sure in the mouth of the next. After the spawn-time in late spring and summer, the food-demand of the mackerels rises strongly. In large swarms, the mackerel then hunt for the brood of the herrings, sprats, cods, whiting and sand sparrows. During this time, mackerels often form huge swarms together with herrings.
The mackerel is a typical schooling fish of the coastal areas. They live in the North Atlantic, off the coastal waters of the north-eastern North America as far as to the west coast of Europe, in the North Sea, Baltic Sea (only sporadically) and in the Mediterranean as far as to the Black Sea.
There are two different stocks of mackerel: one in the north-east Atlantic and one in the north-west Atlantic. The two stocks are genetically well distinguishable and there is no mixing.
The stock in the North-East Atlantic spawns in Spanish and Portuguese waters, in the Bay of Biscay, off Ireland/Northwest UK and in the Skagerrak (North Sea). The stock of the Northwest Atlantic stretches north to Labrador (Canada) and south to Cape Lookout, North Carolina (USA).
The mackerel stock in the Northeast Atlantic is fished on a lot but still it seems to be healthy. The stocks in the Mediterranean and middle Eastern Atlantic cannot be estimated, because of a lack of data. For the West Atlantic stock, some regions are overfished, and in others the situation is uncertain.
Fisheries & Aquaculture
This is a species with high economic importance. Mackerel is managed by a minimum landing size and a maximum catch quota. In the targeted fishery for mackerel there is little by-catch, e.g. the mackerel fishery with pelagic trawling and purse seines has little by-catch and the seabed is not damaged at all but nevertheless whales or dolphins can land in the fishing nets, but since mackerel occurs in large swarms and the catch quota is limited, large quantities of mackerel itself end up as by-catch. Gillnets can catch dolphins, sea turtles, sharks and seabirds. Hand fishing is very selective and has almost no by-catch. Because schools of mackerel are very high quality and energy-rich food sources for predatory fish, marine mammals and seabirds, i.e. overfishing is a high nuisance to the marine ecosystem.